How does a metal fence affect police radar?

Answer 1

It depends on what the wavelength of the radar is and the size of the hole in say a chain link fence. If the holes are smaller then a quarter wavelength of the radar's RF carrier then it will appear as a solid surface to the radar. This may mean that the wire would have to be quite small to have this effect for radar speed guns. If the fence is just wire and posts then and the fence was perpendicular to the radar beam it is possible that a saturation could occur and this saturation would invalidate the Doppler measurement. However radar's tend to be set up to use only 1 quarter of the dynamic range to allow for saturations, such that they do not invalidate the measurements. To prove a case in law you would be making a technical case that either the radar cross section of your vehicle was smaller then that of another vehicle's at the same range travelling the same direction or that the measurement made was invalidated by a multi path effect, that included two vehicles Doppler motion. Perhaps the presents of the chain link fence with holes smaller then the 1 quarter wavelength provided the surface for the multipart effect and that the measurement from your vehicles passed to the fence with a larger cross section then the direct path, based on the shape of your vehicle. It is fairly typical that cars have a large cross-sectional area from the rear then from the front. You will need to get the technical parameter you would need to prove this and would mean some level of cooperation for the speed gun manufacture. However I think that speed guns tend to be quite high frequency as the aperture sizes are small so the beam width could be fairly narrow. You could try to make the case that the main lobe peak power point was miss-aimed at the fence rather then your car, therefore providing a higher gain for the indirect path rather then the direct path and that this path therefore included inter-modulations from the fence and other vehicles in that path that dominated the phase content from the direct path to and from your vehicle.